1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image by scanning a sensitized material with the laser beam modulated by an image signal.
Also, the present invention relates to a constant current circuit switching device suitably used for a semiconductor laser driving device which emits the laser beam modulated with the image signal.
2. Related Background Art
FIG. 1 is a circuit block diagram for explaining a constitution of a conventional image forming device, and its constitution and operation will be described below.
Image data expanded in an image processor 101 is transferred to a controller 102 as an image signal 104. The controller 102 performs a sequence of forming an image, as well as controlling the conveyance of printer sheets.
The controller 102 indicates the luminous energy necessary for forming the image to a laser control unit 103. This signal is a luminous energy indication signal 106. The controller 102 monitors whether or not the laser beam is emitted with a necessary luminous energy, using a luminous energy monitor signal 107, thereby raising or lowering the luminous energy indication signal 106.
The laser control unit 103 lights up a laser device with a driving current in accordance with the luminous energy indication signal 106 from the controller 102. This light-up is made at a time when a laser light-up signal 105 becomes true. The laser control unit 103 transfers the laser luminous energy to the controller 102 as the luminous energy monitor signal 107.
The image processor 101 and the controller 102 are explained as separate units, but may be constructed as the same unit.
FIG. 2 is a detail block diagram for explaining the constitution of controller 102 as shown in FIG. 1, wherein a conveyance control unit is omitted. Note that 111 is an image control signal generator.
The image signal 104 transmitted from the image processor 101 serves as an ON/OFF signal for the laser to form the image. However, the range for forming the image (range where the image signal is effective) is only within an effective region 116 as shown in FIG. 3.
FIG. 3 is an explanation view for explaining the relative relation between effective imaging region and image signal.
A slanting line portion 114 within a transfer paper 113 (paper on which the image is formed) is a region for forming the image.
Now, if the laser beam is scanning with the laser light-up signal 115, the image signal 117 at this time can be made true only within the effective region 116.
It is not necessarily assured that the image signal 117 is not made true outside of the effective region 116, because the image processor 101 is different for each apparatus. Thus, the controller 102 must be masked so that the laser light-up signal 115 does not become true outside of the effective region 116. That signal is an image formation authorized signal 109.
Also, the controller 102 outputs the luminous energy indication signal 106 in accordance with the luminous energy monitor signal 107. In this case, the laser must be forcedly caused to make a continuous emission, even if out of the effective region.
To this end, the controller 102 makes a forced light-up signal 110 true to cause the laser to make the forced emission. To form the image in this way, at least three signals, are required, namely image signal 104, image formation authorized signal 109 and forced light-up signal 110.
FIG. 4 is a circuit block diagram for explaining a detailed constitution of the laser control unit 103 as shown in FIG. 1.
The laser control unit 103, which consists of a constant current source 118, a switch 119, a laser diode 120, and a photodiode 121 for monitoring the laser luminous energy, causes the laser to emit the light with the luminous energy in accordance with the luminous energy indication signal 106. At this time, it is the laser light-up signal 105 that serves as the ON/OFF signal. Also, the luminous energy monitor signal 107 for controlling the luminous energy is created.
In the following, the operation of each unit will be described in detail.
The constant current unit 118 causes current I to follow in accordance with the luminous energy indication signal 106. This current I is caused to flow through the laser diode 120 or to the ground GND, by the switch 119.
The switch 119 acts to flow the current through the laser diode 120 or directly through to the ground GND in accordance with the state of the laser light-up signal 105. Laser luminous energy can be detected by the photodiode 121 as the current value. This current value is converted into the voltage value by a loading resistor 122 to become the luminous energy monitor signal 107.
However, in a conventional image forming apparatus, if the image signal has a higher frequency, there was a problem that the waveform was made irregular so that a bad influence was exerted on the image, e.g., image disorder might arise.
For example, in a conventional binary image, the influence of irregular waveforms exerted on the image is small because the frequency of the image signal is relatively low, whereas when an image signal containing high frequency components such as the half tone image is processed, the influence on the image is remarkable. That is, when dealing with the high frequency image signal, it is necessary to consider the image signal line as the transfer line, whereas in a conventional transfer processing, there was a problem that irregular waveforms caused due to the reflection and other factors might appear as the unevenness of density or noise such as a moire pattern on the image.
Also, in a conventional laser control unit, a high frequency image signal is transferred from the image processor 101 via a line for the image signal 104 through logical circuits 112, 108, and input via a line for the laser light-up signal 105 into the laser control unit 103. On this way, there are various factors potentially making the waveform irregular.
Though there was no problem in conventional low frequency image signals, the ratio of irregular waveforms per dot is not negligible in high-speed or high resolution machines. Particularly, when the density is represented by the size of dot (pulse width), there was a significant problem that the irregular duty ratio for image signal might exert a great influence on the density.
Moreover, in the above conventional laser control unit, there was a problem that owing to dispersion in the characteristics of switching transistor constituting the switch 119 and the current or voltage characteristics of semiconductor laser 120, the output of semiconductor laser might not coincide with the duty of the driving signal, when driven at a certain duty by the laser light-up signal 105.
In the following, this problem will be described with reference to FIGS. 5 and 6.
FIG. 5 is a circuit diagram for specifically showing the constitution corresponding to the constant current source 118, the switch 119, the semiconductor laser (laser diode) 120, the optical detector (photodiode) 121 and so on as shown in FIG. 4. FIG. 6 is a characteristic view representing one example of the current and voltage characteristics of semiconductor laser, indicating that the forward voltage is differently shown with the same current flow due to dispersion of each laser diode.
In FIG. 5, a constant current setting section consists of an operational amplifier 201, and an npn transistor 202 for receiving its output at the base and setting the constant voltage along with a resistor 210. A driving circuit is formed of a pnp current mirror component consisting of pnp transistors 203-205, and an npn current mirror component consisting of npn transistors 206-208 to which the constant current I.sub.ref is supplied by the pnp current mirror component. From the npn current mirror component, the constant current is supplied to the semiconductor laser 23. Resistors 211, 212 are for self-saturation, and a resistor 213 is for bias stabilization in the pnp current mirror component.
The switch 119 is formed of a level conversion circuit for receiving a modulated signal VD (corresponding to the laser light-up signal 105) and an npn transistor 209 having its output applied to the base for switching the constant current to the semiconductor laser 223.
The optical detector and APC circuit 214 is one provided with an optical sensor for detecting the light intensity of semiconductor laser 223, and means for giving the potential to the + input of the operational amplifier so as to keep the light intensity of semiconductor laser 223 constant with its output. 215 is a power line connectable to the power supply.
In the above circuit, considering the operation of the switching npn transistor 209, the npn transistor 209 operates in the breaking or saturation region to turn on/off the semiconductor laser 223, wherein there is such a property that the transistor lying in the saturation region continues to be in the on state until carriers stored in the base region due to the so-called storage effect of minority carrier disappear due to discharge. Accordingly, for the modulated signal VD, the semiconductor laser 223 has a shorter light emitting time by the period during which the transistor continues the on state. That is, there is a problem that the VD signal and the duty do not coincide. This behavior is shown in FIG. 7. As shown in the same figure, a semiconductor laser anode voltage signal causes a delay by the delay time t.sub.pd with respect to the modulated signal VD. Next, the operation of the pnp transistor for supplying the driving voltage will be described. If the switching npn transistor exits from the saturation state and enters the breaking state, a delay will occur by the time when the current flows through the semiconductor laser 223. This delay is the time t for charging the parasitic capacitance caused by mainly a collector of the pnp transistor 205, when the collector potential of the pnp transistor 205 is caused to rise by the amount of forward voltage of the semiconductor laser, wherein provided that the parasitic capacitance is C.sub.C, the constant current flowing to the collector of the pnp transistor 205 is I.sub.ref, and the forward voltage of the semiconductor laser is V.sub.F, EQU C.sub.C .multidot.V.sub.F =I.sub.ref .multidot.t
i.e., EQU t=C.sub.C .multidot.V.sub.F /I.sub.ref
From the above expression, it will be found that the delay time t depends on the forward voltage V.sub.F of the semiconductor laser.
This behavior is shown in FIG. 8. A further detailed explanation of FIG. 8 is as follows. Assuming that the saturation voltage of the npn transistor 209 is V.sub.CE 209.sub.(sat) and the base-emitter voltages of the npn transistors 206, 207 are V.sub.BE 206, V.sub.BE 207, respectively, the L.sub.O level in collector potential waveform of the pnp transistor 205 of FIG. 8 is EQU V.sub.CE 209.sub.(sat) +V.sub.BE 206+V.sub.BE 207,
which is a value irrespective of V.sub.F of the semiconductor laser. Upon rising by V.sub.F above this potential, the semiconductor laser 223 will emit the light, but the charging time t becomes longer as indicated in the above expression, along with the increase of V.sub.F, due to dispersion of V.sub.F as shown in FIG. 6, and as a result, there is a problem that the laser current waveform is made shorter.
As described above, for the modulated signal VD, there are factors of putting the duty out of order such as the saturation of the switching transistor, and the fluctuation of collector voltage of the pnp transistor for driving the constant current, causing a problem which cannot be avoided in conventional examples, so that the boldness or gradation of print line can not be output properly.